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    <title>Document</title>
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<script>
    /**
     * unshift(被添加元素) push(被添加元素)
     */
    var currentDay = "星期四";
    var dayList = [currentDay];
    dayList.unshift("");
    dayList.push("");

    var arr1 = ["hello","world"];
    var arr2 = ["hello","Javascript"];
    var arr3 = ["arr3"];

    var arr4 = arr1.concat(arr2);
    console.log(arr4);
    /**
     * arr.concat(arr1,arr2)
     * 会将arr，arr2，arr1合并得到一个新的数组
     * 但是不会对arr产生影响
     */
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